Rescuers said many of those who died were sleeping when flames toxic smoke swept through the hospital's orthopaedic ward and intensive care units, while some suffocated because they were trapped by locked doors and windows.

Police arrested six hospital officials on charges of culpable homicide. West Bengal's Chief Minister Mamata Bannerjee confirmed a criminal investigation had begun and said the owners and management had committed "an unforgivable crime".

They will face "the harshest punishment possible," she said, adding that the hospital's licence to practice had been cancelled.

Officials said 73 people died at the city's private AMRI hospital, with another 16 succumbing to their injuries later. Fears were voiced of a radiation leak after the fire damaged its radiology and oncology department.

Survivors described being alerted to the fire by screams, staff indifference and scenes of panic as they tried to escape from a sealed, air-conditioned and locked building without fire escapes.

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Television pictures showed frail bandaged patients trussed in rope with faces blackened by smoke as they were lowered to safety by firemen.

One man said his 36 year old wife, who was being treated in the hospital for a broken ankle, had phoned him shortly before she died to tell him she could not breathe. She said some of the patients were hobbled by injuries and could not escape.

Sudipto Nandy, whose 32-year-old brother-in-law died in the tragedy, said he blamed the hospital, which denies any violations of safety measures, for his death. “It all ended with such a horrible death. They had locked the hospital from inside preventing the people from going in for rescue,” he said.

Firhad Hakim, West Bengal's Urban Development minister who was at the hospital during the fire, said he was called by a friend who asked him to rescue his father who was trapped inside. "I could not do anything. I saw his burnt body," he said.

Sanjeet Kayal, a nearby resident, described "helpless patients inside trying to catch our attention, asking for help by pressing their hands and faces against windows." Another said their efforts were frustrated because all doors and windows were locked.

Ms Bannerjee's visit to the hospital provoked anger among patients' relatives waiting outside who said her convoy had blocked ambulances rushing survivors to another hospital.

Officials said the fire had started at 3.30am in the seven story building's basement, which had originally been a car park but was used to store highly flammable materials. Flames spread up through the floors and thick toxic smoke seeped throughout the building through air conditioning and ventilation shafts.

Sagarika, who was staying at the hospital with a relative, criticised hospital staff who failed to help and tried to stop volunteers coming to their rescue. "We were sleeping, then we heard someone screaming that there was a fire. The hospital staff told us not to worry, nothing will happen.

"People from nearby slums tried to help but the authorities did not allow them. Instead those who wanted to rescue patients were harassed. Had they not helped, half the patients in the building would have died," she said.

Prakteek Biswas, who was staying at the hospital where his mother was a patient, said he was woken by "cries from outside the ward." "Within no time the chaos grew and people started to run for safety, except for the patients. The patients were told not to panic by the staff. Within minutes I saw flames and smoke entering the third floor. I managed to take my mother to safety. Most of the patients where in their beds with no staff around to help them," he said.

One volunteer rescuer, Sudip Das, who helped to smash open windows to help patients to safety, said inside the building was dark and full of smoke it was impossible to see. "We had to break the glass of the doors and panes to get inside in order to rescue the patients. We found they were unconscious," he said.

One fortunate patient described how he had learned of the fire when he got up to go to the lavatory in the night. "I heard nurses saying that a portion of the building had caught fire. But they did not help me. Finally along with some other patients I came to a window. The fire brigade personnel came and rescued me by breaking open the windows," he said.

A hospital spokesman earlier said 160 patients had been in the 200 bed block when the fire broke out, and as many as 50 in the Intensive Care Unit. "Eighty per cent have been rescued," he said in a claim which turned out not to be true.

One man told Agence France Presse he feared for his mother who was still inside the hospital. "My mother is in the intensive care unit. She's 70 years old. I don't know if she is alive or not," said Khokon Chakravathi.